This invention relates to a fluid dynamic bearing for use in polygonal motors and other rotational drives in hard disk drive units.
Fluid dynamic bearings for use in drives of high speed rotation as typified by hard disk drive units are generally constructed by fitting a shaft in a concentric sleeve for rotation and filling the gap therebetween with oil. The sleeve or shaft is provided with a radial pattern of herringbone channels, for example. Upon high-speed rotation of the shaft, a pressure distribution is created in the oil between the sleeve and the shaft whereby the shaft is kept afloat.
To prevent run-out, the shaft requires roundness, which requirement becomes important for high-speed rotation.
In the fluid dynamic bearing, no contact occurs in the steady state. During unstable low-speed rotation upon start-up and shutdown, however, contact can occur between the sleeve and the shaft, which are abraded. Then metal chips formed by abrasion and debris are admixed into the oil to change the viscosity thereof, resulting in a loss of performance or a reduced lifetime.
To overcome such problems, JP-A 7-37233 discloses a rotating drum supported by a fluid dynamic bearing in which grooves for generating a pressure are formed by a hard protective film, preferably a diamond-like thin film. Also JP-A 11-62947 discloses a fluid dynamic bearing comprising a shaft and a rotating sleeve which are provided on the surface with a carbon protective film, preferably of amorphous carbon or diamond-like carbon. In the preferred embodiment, the shaft is provided with grooves for generating a dynamic pressure, only the surface of the grooves is not covered with the carbon protective film, and the remaining surface of the shaft and the surface of the rotating sleeve are covered with the carbon protective film. Further, JP-A 6-241232 discloses a fluid dynamic bearing comprising a first bearing member formed to a cylindrical shape, a second bearing member through which the first bearing member is inserted and which has an inner surface opposed to the outer surface of the first bearing member at a predetermined spacing, and means disposed on the outer surface of the first bearing member or the inner surface of the second bearing member for generating a dynamic pressure. The dynamic pressure generating means generates a dynamic pressure between the outer and inner surfaces of the first and second bearing members for providing radial support. At least one of the outer surface of the first bearing member and the inner surface of the second bearing member is provided with a diamond grit cluster thin film.
Although the above-referred JP-A 7-37233 does not describe the detail about the formation of the pressure generating pattern by the hard protective film, two separate steps are necessary for forming the hard protective film and for forming the pattern. It is regarded appropriate to effect the patterning step by (a) photoresist technique or (b) laser etching. The photoresist technique (a) generally involves a series of steps of resist coatingxe2x86x92dryingxe2x86x92exposurexe2x86x92development xe2x86x92etchingxe2x86x92resist removal. One typical laser etching technique (b) is by scribing with a laser beam a portion of a hard protective film to be patterned and removing that portion (see Japanese Patent Application No. 10-103414).
A substantial cost is needed when a hard protective film is formed and patterned by either of the above techniques. Additionally, since only the pressure-generating pattern is formed by the hard protective film, abrasion can occur on the bare portions of the members where no hard protective film is formed. Further, if the dynamic pressure-generating grooves are as deep as several microns to several tens of microns, a film having a corresponding thickness (of several microns to several tens of microns) must be coated, which is cumbersome. The coated film tends to peel off because of internal stresses.
With respect to JP-A 11-62947, in order that the dynamic pressure-generating groove portion be not covered with the carbon protective film, the dynamic pressure-generating groove portion is masked with a photoresist so that no amorphous carbon may deposit on that portion, and the photoresist is later removed with acetone. When the carbon protective film is formed and patterned by this process, the number of steps is increased as well as the cost. Abrasion can occur on those portions where the carbon protective film of amorphous carbon is not formed.
The method of JP-A 6-241232 is to form a uniform diamond grit cluster thin film on a bearing member by uniformly dispersing diamond cluster grits of about 10 nm (100 A) in a plating solution, and codepositing the grits along with the plating material on a surface of the bearing member to be covered. The diamond grit cluster thin film formed by this method includes portions of diamond cluster grits and portions consisting solely of the plating material, of which the plating material portions can react with the metal of the associated member in contact therewith, causing corrosion and abrasion. As the plating is gradually worn away, the released chips are admixed into the oil along with diamond cluster grits whereby the oil is contaminated, leading to a shorter lifetime.
An object of the invention is to provide a fluid dynamic bearing comprising a shaft and a sleeve which has improved wear resistance and durability, maintains the roundness of the shaft unchanged over a long term of operation, experiences no loss of rotational precision by repeated operation, and is easy to manufacture.
According to the invention, there is provided a fluid dynamic bearing comprising a pair of members engaged for rotation with a fluid filled therebetween, the members bearing a concave and/or convex pattern consisting of grooves or protrusions and lands for generating a pressure distribution in the fluid, at least one of the pattern-bearing members being covered with a diamond-like carbon film. The at least one member is first formed with the pattern and then covered with the diamond-like carbon film over both the surface of the grooves or protrusions and the surface of the lands.
Preferably the diamond-like carbon film has a basic composition represented by CHxSiyOzNvFw wherein x, y, z, v and w representative of molar ratios of the associated elements are in the range: 0.05xe2x89xa6xxe2x89xa60.7, 0xe2x89xa6yxe2x89xa63.0, 0xe2x89xa6zxe2x89xa61.0, 0xe2x89xa6vxe2x89xa61.0, and 0xe2x89xa6wxe2x89xa60.2.
In one preferred embodiment, the fluid dynamic bearing has a shaft fitted in a sleeve for rotation with a fluid filled therebetween so that when the shaft rotates within the sleeve, a pressure distribution is generated in the fluid between the shaft and the sleeve for keeping the shaft afloat, the pattern-bearing member covered with a diamond-like carbon film is the shaft or the sleeve, which is first formed with the pattern and then covered with the diamond-like carbon film over the entire pattern-bearing region. Preferably the shaft has a collar opposed to one end portion of the sleeve and a shank fitted in the sleeve and having a surface in contact with the fluid, the collar has a thrust surface opposed to one end portion of the sleeve and provided with the pattern, and the thrust surface of the collar and the surface of the shank in contact with the fluid are covered with the diamond-like carbon film. Further preferably, the sleeve has a cylindrical inner wall surface in contact with the fluid, which is provided with the pattern, and the one end portion of the sleeve opposed to the collar of the shaft and the inner wall surface of the sleeve are covered with the diamond-like carbon film.
Typically the fluid is an oil or air or gas.